coredump: restore compatibility with older patterns

This was broken in f45b801551. Unfortunately
the review does not talk about backward compatibility at all. There are
two places where it matters:
- During upgrades, the replacement of kernel.core_pattern is asynchronous.
  For example, during rpm upgrades, it would be updated a post-transaction
  file trigger. In other scenarios, the update might only happen after
  reboot. We have a potentially long window where the old pattern is in
  place. We need to capture coredumps during upgrades too.
- With --backtrace. The interface of --backtrace, in hindsight, is not
  great. But there are users of --backtrace which were written to use
  a specific set of arguments, and we can't just break compatiblity.
  One example is systemd-coredump-python, but there are also reports of
  users using --backtrace to generate coredump logs.

Thus, we require the original set of args, and will use the additional args if
found.

A test is added to verify that --backtrace works with and without the optional
args.

test/units/TEST-87-AUX-UTILS-VM.coredump.sh

@@ -191,14 +191,18 @@ rm -f /tmp/core.{output,redirected}
 (! "${UNPRIV_CMD[@]}" coredumpctl dump "$CORE_TEST_BIN" >/dev/null)
 
 # --backtrace mode
-# Pass one of the existing journal coredump records to systemd-coredump and
-# use our PID as the source to make matching the coredump later easier
-# systemd-coredump args: PID UID GID SIGNUM TIMESTAMP CORE_SOFT_RLIMIT HOSTNAME
+# Pass one of the existing journal coredump records to systemd-coredump.
+# Use our PID as the source to be able to create a PIDFD and to make matching easier.
+# systemd-coredump args: PID UID GID SIGNUM TIMESTAMP CORE_SOFT_RLIMIT [HOSTNAME]
 journalctl -b -n 1 --output=export --output-fields=MESSAGE,COREDUMP COREDUMP_EXE="/usr/bin/test-dump" |
-    /usr/lib/systemd/systemd-coredump --backtrace $$ 0 0 6 1679509994 12345 mymachine
-# Wait a bit for the coredump to get processed
-timeout 30 bash -c "while [[ \$(coredumpctl list -q --no-legend $$ | wc -l) -eq 0 ]]; do sleep 1; done"
-coredumpctl info "$$"
+    /usr/lib/systemd/systemd-coredump --backtrace $$ 0 0 6 1679509900 12345
+journalctl -b -n 1 --output=export --output-fields=MESSAGE,COREDUMP COREDUMP_EXE="/usr/bin/test-dump" |
+    /usr/lib/systemd/systemd-coredump --backtrace $$ 0 0 6 1679509901 12345 mymachine
+# Wait a bit for the coredumps to get processed
+timeout 30 bash -c "while [[ \$(coredumpctl list -q --no-legend $$ | wc -l) -lt 2 ]]; do sleep 1; done"
+coredumpctl info $$
+coredumpctl info COREDUMP_TIMESTAMP=1679509900000000
+coredumpctl info COREDUMP_TIMESTAMP=1679509901000000
 coredumpctl info COREDUMP_HOSTNAME="mymachine"
 
 # This used to cause a stack overflow